This invention relates to positive displacement pumps for delivering a fluid in an accurate and reproducible manner. More particularly, the invention relates to a positive displacement pump that delivers fuel and/or water to a fuel cell which utilizes direct oxidation of a hydrogen-containing fuel for the production of electricity.
Over 150 years ago a British physicist conceived of the first fuel cell consisting of an electrochemical reaction of hydrogen and oxygen to produce electricity and water. The electrochemical reaction in the reverse direction is the subject of many secondary education science experiments. In these experiments, the student passes an electrical current through a beaker of water to split water molecules thereby producing hydrogen gas and oxygen gas in a two to one molar ratio. A fuel cell reverses this secondary education learning experience. Simply put, combining hydrogen (a fuel) with oxygen (an oxidant) under the proper conditions, yields water and an electrical charge.
The basic chemical equation reveals the most attractive feature of electrical power production using fuel cell technology. The only byproducts of fuel cells that use hydrogen as the fuel and oxygen as the oxidant are water and electricity. The environmental advantages of using such a fuel cell to generate electrical power are readily apparent. Furthermore, relative to internal combustion engines, fuel cells produce very little waste heat and do not need to "idle" thereby operating more efficiently than internal combustion engines.
Of course, implementation of fuel cell technology in an economical and practical manner has proved difficult. The air around us contains an abundant supply of oxygen for use as the oxidant. Providing suitable hydrogen fuel, however, is recognized as a primary hurdle facing commercial realization of fuel cell technology, especially in vehicles. Storing pure hydrogen gas or liquid on board a vehicle or at vehicle filling stations is unfeasible at this time.
A promising solution to the hydrogen storage problem is the use of an organic compound such as methanol (CH.sub.4 O; CH.sub.3 OH) as the fuel. The methanol fuel is then chemically treated or "reformed" to increase the percentage of hydrogen within the fuel before introducing it into the fuel cell. Many organic compounds are suitable as fuels, including many hydrocarbons and other compounds. Although using such fuels increases the environmentally harmful emissions from the fuel cell, these emissions remain an order of magnitude or two below that of internal combustion engines and still at least half that of battery-powered vehicles (when emissions from generation of power to charge the battery are included).
One difficulty with using organic compound fuels is delivering or pumping the fuel through the reforming process and then to the fuel cell. When starting with a liquid fuel such as methanol, the reforming process includes vaporizing the fuel and then introducing the fuel to a catalyst to strip out carbon and oxygen molecules. The reforming process requires that the fuel be pumped into a vaporizer at a high pressure that is directly related to the flow rate of fuel through the reformer. Additionally, the reforming process produces random pressure fluctuations on top of the flow rate-related pressure. Furthermore, in order to optimize efficiency of the fuel cell, it is very important to deliver precise quantities and flow rates of the fuel to the fuel cell. Otherwise, the fuel will be wasted because there will be either too much or too little oxidant in the fuel cell with which to react. Finally, a fluid delivery system should have a short response time for changes in fuel flow rate in order to adequately respond to the power demands of a typical vehicle.
Water circulation is another difficulty with fuel cells that use organic-compound fuels. In addition to being a byproduct of the electrochemical reaction at the fuel cell, water may be used as an additive to or humidifier of the fuel (see, e.g., U.S. Pat. No. 5,573,866 to Van Dine et al.), as a coolant in the fuel cell (see, e.g., U.S. Pat. No. 5,503,944 to Meyer et al.) and as an humidifier of the oxidant-air supply (see, e.g., U.S. Pat. No. 5,366,818 to Wilkinson et al.). Thus, it is important to provide a fluid delivery system that is compatible with both organic solvents and water.
Traditional systems of delivering fuel, such as a fuel injector used with an internal combustion engine, have proved inadequate for use in a fuel cell with a reformer process In general, these types of systems do not respond well or quickly to a varying and random back pressure such as that produced by a reformer in a fuel cell system. In particular, and among other difficulties, there are three basic shortcomings present in fuel injectors and related systems.
First, a fuel injection system depends upon maintaining precise pressures and pressure differentials within the system. Thus, a relatively complicated and expensive pressure regulating mechanism (often including a booster pump and return fuel line) is required within the fuel injection system. Second, the fuel injector system cannot provide reproducible and accurate fuel delivery rates against a variable and/or random back pressure. The fuel injector system delivers fuel by opening a fuel injector valve. A typical fuel injector valve operates electro-mechanically and opens upon a signal from a fuel injector controller. Other fuel injector valves respond to changes in fuel line pressure and upon a sharp increase line pressure, the valve opens and delivers fuel to the engine. In either case, however, the pressure regulator is too slow to respond satisfactorily, and in the face of variable and random back pressure (such as that from a vaporizer or reformer), a constant pressure differential across the valve is difficult to maintain. Without a constant pressure differential, the flow rate from the valve will be uneven. Thus, fuel injection systems cannot provide reproducible and accurate fuel delivery rates against a variable and random back pressure. The third difficulty is that fuel injection systems are incompatible with water and will corrode when exposed to water based solutions.
In view of the foregoing, an object of the invention is to provide an improved apparatus for delivering fluid to a fuel cell and a reforming process.
Another object of the invention is to provide such apparatus to deliver fluid to the fuel cell against a back pressure that is flow-rate dependent.
Yet another object of the invention is to provide such apparatus to deliver fluid to the fuel cell against a back pressure that is randomly variable.
Yet still another object of the invention is to provide such apparatus to deliver fluid to the fuel cell at an accurate and reproducible flow rate against a variable and fluctuating back pressure.
An additional object of the invention is to provide such apparatus to deliver fluid to the fuel cell with a short response time for flow rate changes.
It is another object of the invention to provide such apparatus for delivering both an organic based fluid and a water based fluid to the fuel cell.
Yet still a further object of the invention is to provide such apparatus as can be implemented inexpensively.